Explanation

Tritium is a beta-emitting radioactive isotope of hydrogen.

Its nucleus consists of one proton and two neutrons,

Making it three times as heavy as a hydrogen nucleus (with its one proton) and one-and-a-half times as heavy as deuterium (which contains one proton and only one neutron).

nucleus of tritium ${}_1^{3}H=1proton+2neutrons$